The use of fiber optic transmission techniques is now well accepted in telephone and data transmission arts. Optical multiplexing to enable each fiber to carry a great multitude of data and voice channels is also well understood. Several applications require the passage of power or communication signals (e.g., voice, video, data streams, etc.) across an interface between a rotating reference frame and a non-rotating or stationary reference frame.
One such application is a turret for a Floating Production, Storage and Offloading System (“FPSO”), a moored ship-shaped facility capable of producing oil from subsea wells and for loading and offloading the oil into shuttle tankers, a common scheme when pipeline infrastructure is not available. A critical part of the FPSO is the turret transfer system that provides the ability for an FPSO to weathervane (i.e., allowing the ship to take the position of least resistance relative to the wind, waves and currents) around the mooring, thus minimizing the loading imposed by the environment. The most common solution is a swivel system with the ship attached to, but free to rotate continuously about, a vertically-disposed mooring that is anchored to the seabed. The swivel includes electrical slip rings to power submerged pumps and other equipment and for transmission of low data rate signals, along with fluid rotary unions for passing certain chemicals used in the oil and gas recovery process, as well as for the pumping the product itself from the subsea wells.
As data transmission rate requirements have increased, to take advantage of multiplexing technologies and to link multiple oil rigs to the FPSO possibly over distances, it has become desirable to include fiber optic communications through the turret system, requiring a fiber optic rotary joint. Multiple fibers are required to tie into separate fields, for redundancy, and to separate various vital and non-vital signals. Passive solutions without regeneration through electrical and electro-optic means are desired for safety, maintenance and reliability reasons. Current FPSO systems commonly require from ten to twenty optical fibers, typically all singlemode, with a desired throughbore diameter of 30 cm or more for fluid passage.
A conventional multiple-fiber optic rotary joint is provided by Focal Technologies of Dartmouth, Nova Scotia, Canada, which allows fifteen or more optical fibers to be feed into and out of the joint (see, e.g., U.S. Pat. No. 4,725,116). Other multiple-channel fiber optic rotary joint have been proposed. A number of these use derotating prisms (see, e.g., U.S. Pat. Nos. 4,109,997 and 5,157,745), but, like the Focal design, all are on-axis designs with optical and mechanical elements occupying the central region proximate the axis of rotation. In many rotating applications, it is desirable to use the central region for other equipment or media. Hence, the hollow-bore requirement, and the need for an off-axis rotary joint.
A number of off-axis fiber optic rotary joints are reported (see, e.g., U.S. Pat. Nos. 4,027,945, 4,525,025, 4,943,137, 4,934,783, 5,297,225 and 6,104,849), but none are suited to the FPSO turret requirement. All display high optical losses that require active regeneration of signals, and are impractical for a multiplicity of singlemode optical fibers.
Another solution for some rotary coupling requirements includes limited rotation schemes; that is, solutions that can only turn in one direction for a limited number of rotations before reversal of direction is required. One example is U.S. Pat. No. 5,078,466, wherein a ribbon containing many optical fibers is allowed to coil and uncoil like a clock spring. A variety of other cable spooling mechanisms are reported (see, e.g., U.S. Pat. Nos. 3,822,834 and 5,921,497) that offer a limited number of turns for winch applications and the like. These are not suited to the intended FPSO application as they would need to be rewound with the intervention of a tugboat or other means to break and reset the connection through manual intervention.
Accordingly, there is a need for an improved off-axis rotary joint that will be less expensive to manufacture, that will be capable of operating in an automatic and unattended manner, and that is particularly suited for use in FPSO applications.